An optical fiber diffraction grating can output light having a specific reflection wavelength upon reception of incident light. Owing to this advantage, a great deal of attention has recently been paid to the optical fiber diffraction grating as an important optical part in a division multiplex transmission wavelength division multiplex optical transmission communication system which multiplexes and transmits optical signals having different wavelengths through one optical fiber.
Such an optical fiber diffraction grating is generally constituted by a coating portion consisting of a plastic material and concentrically surrounding an optical fiber glass portion in which an optical fiber diffraction grating portion having a diffraction grating portion with a core the refractive index of which changes along the optical axis is formed. This coating portion is formed to protect the surface of the optical fiber glass portion, and is generally constituted by a primary coating portion consisting of a UV (ultraviolet) curing resin, a silicone resin, or the like and directly surrounding the optical fiber glass, and a secondary coating portion consisting of a polyethylene resin or the like and surrounding the lower coating portion. In addition, in order to improve the strength and hydrogen resistance characteristics, the primary coating directly surrounding the glass portion may have a two-layer structure using a carbon coating as the lower layer.
In the optical fiber diffraction grating having this structure, since the optical fiber has a positive linear thermal expansion coefficient within the operating temperature range, when the ambient temperature changes, the optical fiber expands/contracts, and stress is exerted on the optical fiber. The refractive index of the optical fiber glass portion therefore changes owing to the photoelastic effect. As a result, the reflection wavelength by the diffraction grating portion changes.
The reflection wavelength instability of this optical fiber diffraction grating with respect to changes in temperature cannot be neglected when the grating is to be used for a wavelength division multiplex optical communication system. In an optical fiber diffraction grating laser using such a conventional optical fiber diffraction grating as an external resonance reflector for the laser, the oscillation wavelength of the laser varies with changes in ambient temperature, posing a problem in terms of stability.
In order to solve such a problem, a method of fixing an optical fiber diffraction grating to an Invar rod or the like exhibiting small changes in temperature has been proposed (refer to G. W. Yoffe et al., "Temperature-compensated optical-fiber Bragg gratings", OFC '95, Technical Digest, W14, pp. 134-135).
More specifically, as shown in FIG. 6, Al brackets 52a and 52b having relatively large thermal expansion coefficients are mounted on the two ends of a 15-cm long Invar rod 50 having a small thermal expansion coefficient. An optical fiber diffraction grating 56 is fixed to the Al brackets 52a and 52b under a predetermined tension with latches 54a and 54b. In this case, a diffraction grating portion 58 of the optical fiber diffraction grating 56 is positioned between the two latches 54a and 54b.
When the ambient temperature rises, the optical fiber diffraction grating 56 fixed with the two latches 54a and 54b acts to expand. At the same time, however, the Al brackets 52a and 52b expand to decrease the distance between the two latches 54a and 54b. For this reason, the two forces in the opposite directions cancel out each other to reduce the stress exerted on the diffraction grating portion 58 of the optical fiber diffraction grating 56. Even if, therefore, the ambient temperature rises, an increase in the reflection wavelength based on the diffraction grating portion 58 can be prevented.
However, in the method of preventing variations in the reflection wavelength by the diffraction grating portion with respect to changes in temperature by fixing the optical fiber diffraction grating to, for example, an Invar rod exhibiting small changes in temperature, the device is mechanically complicated and hence difficult to handle. It is therefore required to realize a method of preventing variations in reflection wavelength with respect to changes in temperature with a simpler arrangement which is easy to handle.
The present invention has been made in consideration of the above situation, and has as its object to provide an optical fiber diffraction grating which ensures the reflection wavelength stability with respect to changes in temperature and improves the reliability with an easy-to-handle, simple arrangement.
It is another object of the present invention to provide an optical fiber diffraction grating manufacturing method which can easily manufacture such an optical fiber diffraction grating.
It is still another object of the present invention to provide an optical fiber diffraction grating laser using an optical fiber diffraction grating as a resonance reflector, which can vary ensure the oscillation wavelength stability with respect to changes in temperature.